1. Field of the Invention
The invention relates to the art of glass-to-metal seals. In particular, the invention relates to electrical components which provide (1) hermeticity, (2) acceptably low resistance to current flow, (3) an acceptably high level of insulative resistance, (4) an acceptably high level of resistance to glass corrosion, (5) ease of metallurgical bonding to other components, (6) resistance to thermal and mechanical stresses, (7) glass wetting, (8) an expansion coefficient suitable for glass-to-metal seals, and (9) an economically competitive product.
2. Description of Related Art
Various alloys and metals have been used to form glass-to-metal seals. Such metals include Pt, Cu, Ni, Fe, Mo, W, Ni, etc. For instance, ASTM alloy designation B540 includes 34-36% Pd, 29-31% Ag, 13.5-14.5% Cu, 9.5-10.5% Au, 9.5-10.5% Pt and 0.6-1.2% Zn and has been used to make glass-to-metal seals.
U.S. Pat. Nos. 2,284,151; 2,446,277; 3,141,753; 3,160,460; 3,199,967; 3,209,103; 3,457,539; 3,637,917; 3,646,405; 3,988,053; 4,103,416; 4,277,716; 4,649,085; 4,657,337; 4,712,085; and 4,737,601 disclose examples of glass-to-metal seals. These patents do not disclose or suggest using Pd-Ru alloys to form glass-to-metal seals.
U.S. Pat. Nos. 1,166,129; 2,300,286; 2,303,402; 2,303,403; 2,471,297; 2,478,225; 3,467,554; and 3,488,172 disclose various Pd alloys having noble metal additions such as Ru. These patents do not disclose or suggest using Pd-Ru alloys to form glass-to-metal seals.
In sensitive electronic circuitry, a major problem of conventional glass-to-metal seals is contamination due to introduction of impurities into devices incorporating such glass-to-metal seals. Such impurities can be introduced during chemical cleaning of leads after the glass-to-metal seal is formed. In defense related applications, such impurities can result in failure or malfunction of equipment used for telecommunications, advanced weaponry, guidance systems, etc. Accordingly, there is a need in the art for a glass-to-metal seal which avoids introduction of impurities into sensitive electronic circuitry.
The art of sealing glass-to-metal has been practiced for a very long time. Its purpose, when used in electric components, is to provide a sealed environment which would prevent deterioration of that component if exposed to air or other destructive atmospheres. For example, the leads sealed into the base of an incandescent light bulb allow electric current to heat up the filament (usually tungsten) to incandescence. If air were to penetrate into the light bulb, the filament would immediately burn out.
To be useful as a component of an electric or electronics assembly (tubes, transistors, relays, switches, etc.), the materials of construction have several requirements:
1) Provide hermeticity (a leak rate of 1.times.10.sup.-10 scc/sec of He under one atmosphere pressure differential would allow a leak of 1/15 of a cubic inch of gas in approximately 300 years). This is a measure of hermeticity and long term component reliability. PA1 2) Provide acceptably low resistance to the flow of current. PA1 3) Provide an acceptably high level of insulative resistance as well as other electrical characteristics between adjacent metal components (i.e., dielectric constant). PA1 4) Provide an acceptably high level of resistance to glass corrosion. PA1 5) Be easily connected into its assembly (i.e., soldering or welding to its mating component). PA1 6) Be capable of withstanding typical thermal and mechanical stresses that it may be exposed to. PA1 7) Be economic to manufacture. PA1 8) The non-glass components should be wettable by the molten glass and have coefficients of expansion suitable for the glass it is used in conjunction with. PA1 1) KOVAR (29% Ni, 17% Co, balance Fe) having a coefficient of expansion of approximately 50.times.10.sup.-7 in/in/.degree.C. which is in the range of expansion coefficients of the glasses used with it. PA1 2) Ni-Fe alloys (approximately 50% Ni, 50% Fe) having a coefficient of expansion of approximately 100.times.10.sup.-7 in/in/.degree.C. PA1 1) Chemicals present in the bath etch are to some degree absorbed into the interstices and pores of the component and are potential contaminants of the device it is intended to protect. PA1 2) If the lead wires are small in diameter, or if there are a lot of leads that can tangle in cleaning or plating, parts must be individually handled, packed, or wired for cleaning and plating. That can be extraordinarily expensive and damaging to fine wire leads. PA1 3) Gold is expensive and introduces problems of its own when soldered to the lead wires (e.g. gold-tin embrittlement). PA1 4) Most importantly, the underlying base material is not usually metallurgically bonded to the layers of plating (typically nickel-iron, nickel plate and copper plate) and solderability problems still abound.
Among the first materials used for glass-metal seals to meet those requirements was pure platinum. Although this met a number of the requirements and is still used to some degree, the cost of platinum coupled with its poor wetting to glass resulting in relatively low hermetic capability inhibits its wide spread use. Copper, which can be oxidized and coated to produce a bond to glass, suffers from a coefficient of expansion so high as to severely limit the glasses it can be bonded to. An attempt to overcome this drawback resulted in the development of DUMET (copper clad to Fe-42% Ni alloy) which lowered the expansion coefficient to a useful range.
Those are examples of materials that have been used, and a wide list including molybdenum, tungsten, nickel, etc., have been attempted with various degrees of problems and success.
By far, when used for terminals, the largest quantity of glass-metal seals used today are the following:
While those materials resolve the problem of hermeticity, and most other electrical and mechanical requirements, they introduce a problem of solderability that has plagued the industry for years--namely soldering the component to its mating part. In order to resolve that problem, it is a common practice to chemically clean the material in one or more acid baths, nickel plate and then gold plate the components.
This solution has several drawbacks:
The inventors of the present invention approached these problems with the following considerations in mind. First, any metallic ingredient of the alloy must not have a stable oxide. Second, the metal must be very difficult to oxidize in typical glass sealing processes. Third, the metal must be capable of soldering directly upon emerging from the glass sealing oven without benefit of aqueous cleaning cycles.
The inventors experimented with glassing directly to palladium. This introduced a bubble problem caused by the outgassing of hydrogen absorbed by the palladium (a well known property of palladium). The inventors discovered that this problem could be resolved by the use of Pd having a relatively low hydrogen content. As pure Pd may be too soft for certain end use applications, the inventors discovered certain hardening agents could be added to the Pd to increase the hardness and strength without adversely affecting its oxidation and solderability properties. Such hardening agents include other precious metals and especially Ru in amounts up to about 10 wt. %. In addition, parts incorporating Pd or Pd alloy terminals can be glassed in an atmosphere suitable for preventing H.sub.2 bubble formation with very acceptable results.
In a preferred embodiment, a typical seal consists of a stainless steel body, glass compatible with the coefficient of expansion of a 5% ruthenium, 95% palladium alloy, and at least one terminal of this alloy.